The invention relates to signal processing for composite television signals.
Color information carried by a composite television (TV) signal is modulated in quadrature upon a subcarrier. The subcarrier has a frequency corresponding to the line scan frequency in a manner that interleaves the color information about the subcarrier between energy spectra of the luminance baseband signal. In color television systems such as NTSC and PAL, the color information comprises luminance (Y) and chrominance (C) information sharing a portion of the total signal bandwidth. Thus, a Y/C separation procedure in the receiving end is required to extract the luminance and chrominance information individually. The luminance and chrominance information of some image areas, especially in image areas such as a motion edge of high frequency luminance, may not be distinguishable due to imperfect encoding techniques. For example, a television demodulator may incorrectly demodulate high frequency luminance information as chrominance information, causing color artifacts on vertical edges. These include color ringing, color smearing, and the display of color rainbows in place of high-frequency gray-scale information.
Control methods and systems for suppressing cross-color artifacts from a quadrature modulated composite television signal are receiving much attention. Examples of cross-color suppression methods are disclosed in U.S. Pat. Nos. 5,305,120, and 6,504,579. In U.S. Pat. No. 5,305,120, cross-color is suppressed in response to motion information, wherein temporal averaging is employed for cross-color suppression. Alternatively, if motion is detected in the television signals, spatial averaging is employed. Comb filtering also is a popular Y/C separation technique for suppressing cross-color artifacts. Unfortunately, typically not all undesirable cross-color artifacts are removed using comb filter processing. In U.S. Pat. No. 6,504,579, Remy Scherrer discloses adopting a horizontal image gradient, a vertical image gradient, and a diagonal image gradient to determine which direction should have greater weighting in comb filtering in an effort to improve performance.